The present invention relates generally to the field of power conversion electronics and more specifically to the field of space vector modulation for multi-level inverters.
A multi-level inverter is an assemblage of electronically controllable switches coupling a multi-level direct current (DC) input bus to an alternating current (AC) output bus. Often, the AC output bus will comprise multiple output phases.
Nominally, the controllable switches are operated either fully open or fully closed. Under such operation, the only instantaneous output voltages the inverter can produce are those levels available at the DC input bus. Rapid switching between different instantaneous output voltages is often used, therefore, to produce an output whose time average is intermediate to the DC input bus levels. When multiple output phases are produced, each switch closure combination produces a pattern of output voltages across the phases which may be thought of as being distributed in space. Hence, the patterns of output voltages are termed “space vectors,” and the rapid switching scheme is known as “space vector modulation” or “space vector pulse width modulation.”
Compared to two-level inverters, multi-level inverters promise many advantages including reduced voltage and current harmonics at relatively lower switching frequencies and reduced voltage rating requirements for the controllable switches. However, the increased number of DC input bus levels implies an increased number of controllable switches. The mapping (switching function) from desired (reference) space vectors to switch closure combinations is typically not unique. Furthermore, with a larger number of controllable switches, injudicious modulation strategies may result in unacceptably high switching losses.
Opportunities exist, therefore, to find new space vector modulation techniques to resolve the ambiguity inherent in the non-uniqueness of the switching function and to efficiently utilize the controllable switches to reduce switching losses.